Moved bulk operations to index level to better support hosted elasticsearch services

I moved bulk operations to the index level. Instead of calling http://server/_bulk it now calls http://server/index/_bulk. 

The reason for this change is to support hosted elasticsearch services, which sometimes do not allow calls at the server level (indexdepot.com for example). 

All specs pass without a change. I left the Server#bulk method in, just in case anybody is using it directly.